A Study On Grain Boundary Plane Distributions And Grain Boundary Inter-connections In 430 Stainless Steel

As an important engineering material,ferritic stainless steel has a very wide range of applications in industrial production.However,this kind of metal also has the problems of intergranular corrosion and precipitation,which seriously affects its service life and causes major economic losses.In this paper,the essence structure of grain boundary itself is the point-cut,it seeks an innovative method to solve the problem of intergranular corrosion of ferritic stainless steel through the idea of grain boundary engineering.Using domestic 430 ferritic stainless steel as the experimental material,the five parameter analysis(FPA)method based on electron back-scattered diffraction(EBSD)and stereology were used to characterize the grain boundary plane distribution.The grain boundary inter-connection(GBIC)was obtained by crystallographic analysis,and the basic characteristics of near singular grain boundaries(NSB)were determined.Subsequently,the effects of annealing temperature,annealing time and room temperature pre-strain on grain boundary plane distribution(GBPD)?GBIC and the percentage of NSB were investigated.The relationship between grain boundary structure and intergranular corrosion properties were analyzed by O-lattice theory.The main results are as follows:After repeated multi-directional hot forging and recrystallization annealing treatment,a 430 stainless steel sample with uniform grain size and random orientation were obtained.The analysis found that the recrystallized grain interfaces are mainly distributed in {0 1 1},and the grain boundaries with {0 1 1}\{0 1 1} inter-connection characteristics have the highest frequency,which is a kind of near singular grain boundary with low energy and easy to form in430 stainless steel.The annealing temperature has a significant effect on GBPD and GBIC of 430 stainless steel.When the multi-directional hot-forged sample was annealed at 840?,the strength of the recrystallization interface distributed on {0 1 1} reaches the maximum value,and the proportion of {0 1 1}/{0 1 1} inter-connection is the highest;As the annealing temperature increases,the proportion of such near-singular grain boundaries increases,and the grains grow significantly,indicating that the formation of {0 1 1}/{0 1 1} inter-connection near-singular grain boundaries is closely related to the migration of grain boundaries.Annealing time has a significant effect on GBPD and GBIC of 430 stainless steel.After the multi-directional hot-forged sample is annealed at 840?,the strength of recrystallization interface distributed on {0 1 1} and the proportion of {0 1 1}/{0 1 1} inter-connection near singular grain boundaries of the samples increase first and then decrease with the increase of annealing time.During the annealing process at 840?,the grain size also increases first and then decreases with the increase of annealing time,which is related to the fact that 840? is close to the phase transition point,and the long-time holding at this temperature(more than 2hours)leads to the diffusion and segregation of C and Cr,which leads to local phase transformation and nucleation.The amount of pre-strain at room temperature has an adverse effect GBPD and GBIC of430 stainless steel.For the sample pre-strained at room temperature and then annealed at 840?,the strength of the grain interface distributed on {0 1 1} and the ratio of {0 1 1}/{0 1 1}inter-connection near singular grain boundaries are inversely proportional to the amount of pre-strain,and the grain size changes in the same way.The analysis suggests that the density of nucleation during the recrystallization process will increase as the amount of deformation increases,and the migration behavior of the grain boundary is restricted to a certain extent,which has a negative effect on the distribution of the grain boundary toward {0 1 1} and the formation of {0 1 1}/{0 1 1} inter-connection near singular grain boundary.The off-line in-situ intergranular corrosion experiments based on EBSD and SEM observations show that {0 1 1}/{0 1 1} inter-connection near singular grain boundaries are corroded shallower and narrower than general random grain boundaries,indicating this type of near singular grain boundaries have higher corrosion resistance than general random grain boundaries.O lattice calculation analysis shows that {0 1 1}/{0 1 1} inter-connection near singular grain boundaries have periodic lattice point matching areas and periodic dislocations.The structural characteristics of the structure are significantly higher than the general random grain boundary,this is the main reason why the corrosion resistance of grain boundary is obviously higher than that of general random grain boundary.At the same time,the study found that the grain boundaries with {0 0 1}/{0 0 1} and {1 1 2}/{1 1 2} inter-connection characteristics also have higher corrosion resistance,but the ratio of these two types of grain boundaries not tall.